从定期异步请求创建 observable

我想要一种将异步方法转换为可观察方法的通用方法。就我而言，我正在处理使用的方法HttpClient从 API 获取数据。

假设我们有方法Task<string> GetSomeData()需要成为一个单一的Observable<string>其中值是作为以下组合生成的：

• 重复定期调用GetSomeData()（例如每 x 秒）
• 手动触发调用GetSomeData()在任何给定时间（例如当用户点击刷新时）。

由于有两种方法可以触发执行GetSomeData()并发可能是一个问题。为了避免这样的要求GetSomeData()是线程安全的，我想限制并发性，以便只有一个线程同时执行该方法。因此，我需要使用某种策略来处理重叠的请求。我制作了一个（某种）弹珠图，试图描述问题和想要的结果

我的直觉告诉我有一个简单的方法可以实现这一目标，所以请给我一些见解:)

这是我到目前为止得到的解决方案。不幸的是它并没有解决并发问题。

    public class ObservableCreationWrapper<T>
    {
        private Subject<Unit> _manualCallsSubject = new Subject<Unit>();
        private Func<Task<T>> _methodToCall;
        private IObservable<T> _manualCalls;

        public IObservable<T> Stream { get; private set; }

        public ObservableCreationWrapper(Func<Task<T>> methodToCall, TimeSpan period)
        {
            _methodToCall = methodToCall;
            _manualCalls = _manualCallsSubject.AsObservable()
                .Select(x => Observable.FromAsync(x => methodToCall()))
                .Merge(1);

            Stream = Observable.FromAsync(() => _methodToCall())
                .DelayRepeat(period)
                .Merge(_manualCalls);
        }

        public void TriggerAdditionalCall()
        {
            _manualCallsSubject.OnNext(Unit.Default);
        }
    }

延迟重复的扩展方法：

static class Extensions
{
    public static IObservable<T> DelayRepeat<T>(this IObservable<T> source, TimeSpan delay) => source
        .Concat(
            Observable.Create<T>(async observer =>
            {
                await Task.Delay(delay);
                observer.OnCompleted();
            }))
        .Repeat();
}

包含生成可观察值的方法的服务示例

class SomeService
{
    private int _ticks = 0;

    public async Task<string> GetSomeValueAsync()
    {
        //Just a hack to dermine if request was triggered manuall or by timer
        var initiatationWay = (new StackTrace()).GetFrame(4).GetMethod().ToString().Contains("System.Threading.CancellationToken") ? "manually" : "by timer";

        //Here we have a data race! We would like to limit access to this method 
        var valueToReturn = $"{_ticks} ({initiatationWay})";

        await Task.Delay(500);
        _ticks += 1; 
        return valueToReturn;
    }
}

像这样使用（会发生数据竞争）：

static async Task Main(string[] args)
{
    //Running this program will yield non deterministic results due to data-race in GetSomeValueAsync
    var someService = new SomeService();
    var stopwatch = Stopwatch.StartNew();
    var observableWrapper = new ObservableCreationWrapper<string>(someService.GetSomeValueAsync, TimeSpan.FromMilliseconds(2000));
    observableWrapper.Stream
        .Take(6)
        .Subscribe(x => 
            {
                Console.WriteLine($"{stopwatch.ElapsedMilliseconds} | Request: {x} fininshed");
            });

    await Task.Delay(4000);
    observableWrapper.TriggerAdditionalCall();
    observableWrapper.TriggerAdditionalCall();
    Console.ReadLine();
}

这是我对这个问题的看法：

Update:通过借鉴 Enigmativity 的想法，我能够大大简化我建议的解决方案answer. The Observable.StartAsync方法自动处理取消的混乱业务，并且可以简单地通过使用SemaphoreSlim.

/// <summary>
/// Creates an observable sequence containing the results of an asynchronous
/// function that is invoked periodically and manually. Overlapping invocations
/// are prevented. Timer ticks that would cause overlapping are ignored.
/// Manual invocations cancel previous invocations, and restart the timer.
/// </summary>
public static IObservable<T> PeriodicAndManual<T>(
    Func<bool, CancellationToken, Task<T>> functionAsync,
    TimeSpan period,
    out Action manualInvocation)
{
    // Arguments validation omitted
    var manualSubject = new Subject<bool>();
    manualInvocation = () => manualSubject.OnNext(true);
    return Observable.Defer(() =>
    {
        var semaphore = new SemaphoreSlim(1, 1); // Ensure no overlapping
        return Observable
            .Interval(period)
            .Select(_ => false) // Not manual
            .Merge(manualSubject)
            .TakeUntil(isManual => isManual) // Stop on first manual
            .Repeat() // ... and restart the timer
            .Prepend(false) // Skip the initial interval delay
            .Select(isManual =>
            {
                if (isManual)
                {
                    // Triggered manually
                    return Observable.StartAsync(async ct =>
                    {
                        await semaphore.WaitAsync(ct);
                        try { return await functionAsync(isManual, ct); }
                        finally { semaphore.Release(); }
                    });
                }
                else if (semaphore.Wait(0))
                {
                    // Triggered by the timer and semaphore acquired synchronously
                    return Observable
                        .StartAsync(ct => functionAsync(isManual, ct))
                        .Finally(() => semaphore.Release());
                }
                return null; // Otherwise ignore the signal
            })
            .Where(op => op != null)
            .Switch(); // Pending operations are unsubscribed and canceled
    });
}

The out Action manualInvocation参数是触发手动调用的机制。

使用示例：

int ticks = 0;
var subscription = PeriodicAndManual(async (isManual, token) =>
{
    var id = $"{++ticks} " + (isManual ? "manual" : "periodic");
    Console.WriteLine($"{DateTime.Now:HH:mm:ss.fff} Begin {id}");
    await Task.Delay(500, token);
    return id;
}, TimeSpan.FromMilliseconds(1000), out var manualInvocation)
.Do(x => Console.WriteLine($"{DateTime.Now:HH:mm:ss.fff} Received #{x}"))
.Subscribe();

await Task.Delay(3200);
manualInvocation();
await Task.Delay(200);
manualInvocation();
await Task.Delay(3200);

subscription.Dispose();

Output:

19:52:43.684 Begin 1 periodic
19:52:44.208 Received #1 periodic
19:52:44.731 Begin 2 periodic
19:52:45.235 Received #2 periodic
19:52:45.729 Begin 3 periodic
19:52:46.232 Received #3 periodic
19:52:46.720 Begin 4 periodic
19:52:46.993 Begin 5 manual
19:52:47.220 Begin 6 manual
19:52:47.723 Received #6 manual
19:52:48.223 Begin 7 periodic
19:52:48.728 Received #7 periodic
19:52:49.227 Begin 8 periodic
19:52:49.730 Received #8 periodic
19:52:50.226 Begin 9 periodic

使用技术Scan和DistinctUntilChanged操作符是为了在上一个异步操作运行时删除元素，借用自this问题。

¹ _{It seems that the Rx library does not handle this messy business satisfactory though, since it just omits disposing of the CancellationTokenSources it creates.}